I went from 245's to 265/70r18's knowing my speedo and odo would be out by around 6.5% according to the math. The other day I had my Garmin GPS in the truck and at 100kph, the GPS showed I was actually doing around 106-107kph. 'Very good', I figured. That was the expected difference. Yesterday I did a highway run and decided to compare the odo. After 10km on the odo, I did 11 on the GPS. After watching it for 50+kms, I confirmed the difference was 10%. But the speedo difference was still ~6.5%.

Could the odo and speedo in the truck be out of sync? If indeed I am getting 10% extra distance versus 6.5%, my gas mileage is not as bad as I thought it was.

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Could the odo and speedo in the truck be out of sync? If indeed I am getting 10% extra distance versus 6.5%, my gas mileage is not as bad as I thought it was.

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I found almost the same thing when I put on my 265/75 tires. The odo was accurate with the OE tires and the speedo was 'optimistic' (read faster than I was actually going). With the new tires, the odo is off and the speedo is almost dead accurate, as calibrated against GPS.

Oh and do GPS distance traveled over a fairly long trip with few or no tight turns to reduce inherent GPS inaccuracy.

Mine is as much as 5 mph off up around 65-70 mph. The problem is that it's not linear...i.e. it's accurate around 30 mph, off by about 2 mph at 45, and off as much as 5 around 65 mph. I actually complained about it under warranty but they said it was within limits...I assume their specification is at like 50 mph where it is in fact within spec.